Normalization procedures using maximum voluntary isometric contractions for the serratus anterior and trapezius muscles during surface EMG analysis.

The serratus anterior and trapezius muscles are considered to be the only upward rotators of the scapula and are very important for normal shoulder function. A variety of methods have been used to produce a maximum voluntary isometric contraction (MVIC) of these muscles for normalization of EMG data. The purpose of this study was to quantify the surface EMG activity of the serratus anterior muscle and the upper, middle, and lower parts of the trapezius during 9 manual muscle tests performed with maximum effort in 30 subjects. It was found that no one muscle test produced a MVIC for all individuals. Therefore, to perform normalization within each subject, it is suggested that the 2 or 3 tests identified in this study that produce high levels of EMG activity for each muscle be performed. The scapular protraction muscle test that is often used to normalize data for the serratus anterior muscle produced relatively low levels of EMG activity and was not found to be an optimal test. Muscle tests in which an attempt was made to de-rotate the scapula from an upwardly rotated position produced much higher levels of EMG activity in the serratus anterior muscle.     

EMG of serratus anterior and trapezius in the chimpanzee: scapular rotators revisited.

The importance of arm-raising has been a major consideration in the functional interpretation of differences in shoulder morphology among species of nonhuman primates. Among the characters that have been associated with enhancement of the arm-raising mechanism in hominoid primates are the relative enlargement of cranial trapezius and caudal serratus anterior, as the main scapular rotators, as well as changes in scapular morphology associated with their improved leverage for scapular rotation. Yet in an EMG study of cranial trapezius and caudal serratus anterior function in the great apes, Tuttle and Basmajian (Yrbk. Phys. Anthropol. 20:491-497, 1977) found these muscles to be essentially inactive during arm-raising. Although Tuttle and Basmajian suggest that the cranial orientation of the glenoid fossa in apes has reduced the demand for scapular rotation during arm-raising, subsequent EMG studies on other primate species suggest that these muscles do play a significant role in arm motion during active locomotion. This paper presents a reexamination of muscle recruitment patterns for trapezius and caudal serratus anterior in the chimpanzee. All but the lowest parts of caudal serratus anterior were found to be highly active during arm-raising motions, justifying earlier morphological interpretations of differences in caudal serratus anterior development. The lowest digitations of this muscle, while inactive during arm-raising, displayed significant activity during suspensory postures and locomotion, presumably to control the tendency of the scapula to shift cranially relative to the rib cage. Cranial trapezius did not appear to be involved in arm-raising; instead, its recruitment was closely tied to head position.     

Validation of surface recordings of the diaphragm response to transcranial magnetic stimulation in humans.

The integrity of the central efferent motor pathways to the diaphragm can be assessed by using transcranial magnetic stimulation to measure the latency of the corresponding motor evoked potentials with surface electrodes. Because transcranial magnetic stimulation does not activate the diaphragm alone, signal contamination is a potential problem. To evaluate this issue, surface diaphragmatic motor-evoked potential latencies were compared with latencies recorded from diaphragm needle in 9 healthy volunteers. Surface latencies of muscles likely to contaminate the diaphragm signals (serratus anterior, pectoralis major, and tranversus abdominis) were also recorded. The latencies in response to nonfocal transcranial stimulation from surface electrodes were not significantly different from the needle ones (17 +/- 1.3 vs. 17.2 +/- 1.1 ms, respectively) but were significantly different from the latencies of the other muscles. In two cases, signal contamination appeared likely (serratus anterior in 1 case, abdominal muscles in 1 case). It is possible to reliably measure the latency of the diaphragm response to transcranial magnetic stimulation with adequately positioned surface electrodes.     

Head and shoulder posture affect scapular mechanics and muscle activity in overhead tasks

Forward head and rounded shoulder posture (FHRSP) is theorized to contribute to alterations in scapular kinematics and muscle activity leading to the development of shoulder pain. However, reported differences in scapular kinematics and muscle activity in those with forward head and rounded shoulder posture are confounded by the presence of shoulder pain. Therefore, the purpose of this study was to compare scapular kinematics and muscle activity in individuals free from shoulder pain, with and without FHRSP. Eighty volunteers were classified as having FHRSP or ideal posture. Scapular kinematics were collected concurrently with muscle activity from the upper and lower trapezius as well as the serratus anterior muscles during a loaded flexion and overhead reaching task using an electromagnetic tracking system and surface electromyography. Separate mixed model analyses of variance were used to compare three-dimensional scapular kinematics and muscle activity during the ascending phases of both tasks. Individuals with FHRSP displayed significantly greater scapular internal rotation with less serratus anterior activity, during both tasks as well as greater scapular upward rotation, anterior tilting during the flexion task when compared with the ideal posture group. These results provide support for the clinical hypothesis that FHRSP impacts shoulder mechanics independent of shoulder pain.     

The use of EMG biofeedback for learning of selective activation of intra-muscular parts within the serratus anterior muscle: A novel approach for rehabilitation of scapular muscle imbalance

Motor control and learning possibilities of scapular muscles are of clinical interest for restoring scapular muscle balance in patients with neck and shoulder disorders. The aim of the study was to investigate whether selective voluntary activation of intra-muscular parts within the serratus anterior can be learned with electromyographical (EMG) biofeedback, and whether the lower serratus anterior and the lower trapezius muscle comprise the lower scapula rotation force couple by synergistic activation. Nine healthy males practiced selective activation of intra-muscular parts within the serratus anterior with visual EMG biofeedback, while the activity of four parts of the serratus anterior and four parts of the trapezius muscle was recorded. One subject was able to selectively activate both the upper and the lower serratus anterior respectively. Moreover, three subjects managed to selectively activate the lower serratus anterior, and two subjects learned to selectively activate the upper serratus anterior. During selective activation of the lower serratus anterior, the activity of this muscle part was 14.4 ± 10.3 times higher than the upper serratus anterior activity (P < 0.05). The corresponding ratio for selective upper serratus vs. lower serratus anterior activity was 6.4 ± 1.7 (P < 0.05). Moreover, selective activation of the lower parts of the serratus anterior evoked 7.7 ± 8.5 times higher synergistic activity of the lower trapezius compared with the upper trapezius (P < 0.05). The learning of complete selective activation of both the lower and the upper serratus anterior of one subject, and selective activation of either the upper or lower serratus anterior by five subjects designates the promising clinical application of EMG biofeedback for restoring scapular muscle balance. The synergistic activation between the lower serratus anterior and the lower trapezius muscle was observed in only a few subjects, and future studies including more subjects are required before conclusions of a lower scapula rotation couple can be drawn.     

Electromyographic analysis of the serratus anterior and trapezius muscles during push-ups on stable and unstable bases in subjects with scapular dyskinesis

The present study was performed to assess the electromyographic activity of the scapular muscles during push-ups on a stable and unstable surface, in subjects with scapular dyskinesis. Muscle activation (upper trapezius [UT]; lower trapezius [LT]; upper serratus anterior [SA_5th]; lower serratus anterior [SA_7th]) and ratios (UT/LT; UT/SA_5th; UT/ SA_7th) levels were determined by surface EMG in 30 asymptomatic men with scapular dyskinesis, during push-up performed on a stable and unstable surface. Multivariate analysis of variance with repeated measures was used for statistical analyses. The unstable surface caused a decrease in the EMG activity of the serratus anterior and an increase in EMG activity of the trapezius (p = 0.001). UT/SA_5th and UT/ SA_7th ratios were higher during unstable push-ups (p = 0.001). The results suggest that, in individuals with scapular dyskinesis, there is increased EMG activity of the trapezius and decreased EMG activity of the serratus anterior in response to an unstable surface. These results suggest that the performance of the push up exercise on an unstable surface may be more favorable to produce higher levels of trapezius activation and lower levels of serratus anterior activation. However, if the goal of the exercise program is the strengthening of the SA muscle, it is suggested to perform the push up on a stable surface.     

The activity pattern of shoulder muscles in subjects with and without subacromial impingement

Altered shoulder muscle activity is frequently believed to be a pathogenetic factor of subacromial impingement (SI) and therapeutic interventions have been directed towards restoring normal motor patterns. Still, there is a lack of scientific evidence regarding the changes in muscle activity in patients with SI. The aim of the study was to determine and compare the activity pattern of the shoulder muscles in subjects with and without SI. Twenty-one subjects with SI and 20 healthy controls were included. Electromyography (EMG) was assessed from eight shoulder muscles from both shoulders during motion. In the symptomatic shoulder, there was a significantly greater EMG activity during abduction in the supraspinatus and latissimus muscles and less activity in serratus anterior compared to the healthy subjects. During external rotation, there was significantly less activity of the infraspinatus and serratus anterior muscles on the symptomatic side compared to the healthy subjects. On the asymptomatic side, the groups showed different muscle activity during external rotation. Our findings of an altered shoulder muscle activity pattern on both the symptomatic and asymptomatic side in patients indicate that the different motor patterns might be a pathogenetic factor of SI, perhaps due to inappropriate neuromuscular strategies affecting both shoulders.     

Unilateral reduction mammaplasty: sculpturing the breast from the undersurface

Various traditional mammaplasty techniques have been suggested for unilateral breast reduction, and an inverted-T incision is still the most popular approach. However, unilaterally performed traditional techniques can rarely provide long-lasting symmetry because the operated and the unoperated breasts react differently to aging, weight changes, and pregnancy. Considerable residual scarring, interference with clinical and mammographic evaluation, and limited versatility are all major drawbacks of traditional procedures. We have performed unilateral mammaplasties on 47 patients with various types of congenital and acquired asymmetries, reducing and sculpturing the breast from the undersurface by means of minimal incisions, always avoiding horizontal scarring in the inframammary crease. Through a vertical infra-areolar incision, the breast is completely detached from the underlying pectoralis fascia and hooked up, thus completely exposing the undersurface of the mammary cone. The breast can thereafter be reshaped according to the size and shape of the contralateral breast by means of a discoid resection and/or selective sectoral removal of excessive subcutaneous tissues; modifications of the basic discoid resection can increase anterior projection of the new breast mound and can change the inclination of the anteroposterior breast axis on the anterior chest wall both on the horizontal and vertical planes. The results show that if criteria for patient selection are carefully respected, the procedure can provide long-lasting symmetry with minimal residual scarring and fully preserve the breast anatomy, function, and vascularization.     

Nipple-areola reconstruction: satisfaction and clinical determinants

After performing a chart review, the authors identified 120 patients who underwent breast cancer-related reconstruction. All charts were evaluated with regard to breast mound reconstruction type, nipple-areola reconstruction type, the interval between breast mound and nipple-areola reconstruction, the number of procedures needed to achieve nipple-areola reconstruction, patient history of radiation therapy, and complications. A questionnaire was then developed and mailed to all of the patients who underwent both breast mound and nipple/areola reconstruction (n = 105) to evaluate their level of satisfaction. Of the 43 patients who returned the questionnaire, 41 completed all portions correctly. The questionnaire evaluated patient satisfaction with breast mound reconstruction; patient satisfaction with nipple-areola reconstruction; what the patient disliked most about the nipple-areola reconstruction; and whether or not the patient would choose to have breast reconstruction again. Several parameters were then tested statistically against the reported patient satisfaction.A review of all patients who underwent breast reconstruction revealed that their breast mound reconstructions were done using either a TRAM flap (59 percent), a latissimus dorsi flap and an implant (19 percent), an expander followed by an implant (9 percent), an implant only (4 percent), or other means (9 percent). The nipple-areola was reconstructed in these patients with either a star flap (36 percent), nipple sharing (10 percent), a keyhole flap (9 percent), a skate flap (9 percent), an S-flap (8 percent), a full-thickness skin graft (6 percent), or by another means (22 percent). The number of procedures needed to achieve nipple-areola reconstruction was either one (in 66 percent of the patients), two (in 32 percent of the patients), or three or more (2 percent of the patients). Eleven percent of the patients experienced the complication of nipple necrosis.Satisfaction with breast mound reconstruction was reported by 81 percent of patients to be excellent/good, by 14 percent of patients to be fair, and by 5 percent of patients to be poor. Reported satisfaction with nipple-areola reconstruction was excellent/good for 64 percent of patients, fair for 22 percent of patients, and poor for 14 percent of patients. The factors patients disliked most about their nipple-areola reconstruction were, in descending order, lack of projection, color match, shape, size, texture, and position. Statistical analysis of the data revealed inferior patient satisfaction when there was a longer interval between breast mound and nipple areola reconstruction (p = 0.003). No significant difference was observed in nipple/areola reconstruction satisfaction ratings when compared with breast mound reconstruction type (p = 0.46), nipple-areola reconstruction type (p = 0.98), and history of radiation therapy (p = 0.23). There was also no significant difference when breast mound reconstruction was compared with technique (p = 0.51) and history of radiation therapy (p = 0.079). Overall, there was a greater satisfaction with breast mound reconstruction than with nipple-areola reconstruction (p = 0.0001).     

The serratus anterior subslip: anatomy and implications for facial and hand reanimation

The ideal donor muscle for facial and hand reanimation has yet to be found. Donor muscles commonly used today, such as the gracilis and pectoralis minor, are limited by bulkiness and the number of force vectors they can provide. In the authors' study of 50 fresh cadaver serratus anterior muscles, they further describe neurovascular anatomy of the muscle slip (i.e., the portion of the muscle that inserts on a rib) and subslip (superficial or deep subdivision of the slip after division along a loose areolar plane). All 260 slips could be separated into a deep and a superficial subslip, yielding a total of 520 subslips. A branch of the serratus artery (a terminal branch of the thoracodorsal artery serving the lower five to seven slips of the muscle) and a branch of the long thoracic nerve were identified for each of these. Deep subslips were thinner than superficial subslips, both at the origin of the slip on the rib periosteum (2.4 mm versus 3.0 mm, p < 0.0001) and centrally at the serratus artery (3.3 mm versus 4.0 mm, p < 0.0001). In addition, the subslips of the most inferior slip were thinner than those of more superior slips, both at the origin of the slip (2.3 mm versus 2.8 mm, p < 0.0001) and at the serratus artery (3.0 mm versus 3.8 mm, p < 0.0001). Fine anastomosing vessels were present between the slips and the subslips. The average number of anastomosing vessels present between adjacent slips was 1.7, and 2.1 anastomosing vessels were present between the subslips of a given slip. Given the thinness of these vessels (all less than 0.2 mm) compared with those of the vascular pedicle of the subslip (mean, 0.7 mm; all greater than 0.4 mm), the authors believe these can be safely divided without compromising subslip vascularity. After division of these vessels, a mean length of 9.6 +/- 1.5 cm is available to allow independent orientation of each subslip. When the serratus muscle flap is separated into its component subslips, a maximum of 10 possible force vectors may be transferred on a single vascular pedicle. Subslips are significantly thinner than donor muscles commonly used today. These two advantages offer the potential for significant functional and aesthetic improvement when the serratus anterior muscle flap is used for face and hand reanimation. Mimetic muscles such as the orbicularis oculi and orbicularis oris could possibly be reconstructed in their proper anatomical positions.     

Integrated breast mound reduction and nipple reconstruction with the wraparound flap.

A new method for nipple reconstruction is described that combines revision of an autologous tissue breast mound with creation of a projecting nipple. The method is applicable only to reconstructed breast mounds that must be reduced or lifted to achieve symmetry with the opposite breast. In this technique, the mound is reduced as if it were a normal breast, using an inverted-T or vertical mammaplasty pattern. In this way, breast projection can be increased and, if necessary, the inframammary fold can be elevated. A rectangular flap is created from skin and subcutaneous tissue that would normally be discarded during the breast reduction, and this flap is wrapped around on itself to form a projecting nipple. This new technique avoids the flattening of the breast mound usually seen after nipple reconstruction because it does not take tissue away from the completed breast mound to make the nipple. In appropriate patients who require reduction in size of their reconstructed breast mound, the wraparound flap nipple reconstruction is worth considering.     

Differential activation of parts of the serratus anterior muscle during push-up variations on stable and unstable bases of support

No studies have examined the effects of an unstable surface on push-up and push-up plus exercises in terms of the two parts of the serratus anterior muscle. We hypothesized that the lower part of the serratus anterior would have greater activity with an unstable surface, which requires stabilizing the scapular position. The present study was performed to investigate the intramuscular differences between parts of the serratus anterior muscle during push-up and push-up plus exercises. Twelve healthy subjects were included in the study. The upper and lower parts of the serratus anterior and upper and lower parts of the trapezius were investigated by surface EMG during four types of exercise. Repeated one-way ANOVA was used for statistical analyses. Maintaining the push-up plus phase caused significant increases in EMG activity of the upper serratus anterior compared with the push-up ascending phase on both of stable and unstable bases (P < 0.05). The lower serratus anterior showed increased activation on an unstable surface, which required more joint stability than did the stable base. Upper trapezius/upper serratus anterior ratio was significantly lower in the PUP than in the PUA phase with both stable and unstable bases of support (P < 0.05).Further studies are required to investigate the intramuscular variation in activation of the serratus anterior during exercises for rehabilitation.     

Altered activity of the serratus anterior during unilateral arm elevation in patients with cervical disorders

Altered activity in the axioscapular muscles is considered to be an important feature in patients with neck pain. The activity of the serratus anterior (SA) and trapezius muscles during arm elevation has not been investigated in these patients. The objectives of this study was to investigate whether there is a pattern of altered activity in the SA and trapezius in patients with insidious onset neck pain (IONP) (n = 22) and whiplash associated disorders (WAD) (n = 27). An asymptomatic group was selected for baseline measurements (n = 23).Surface electromyography was used to measure the onset of muscle activation and duration of muscle activity of the SA as well as the upper, middle, and lower trapezius during unilateral arm elevation in the three subject groups. Both arms were tested.With no interaction, the main effect for the onset of muscle activation and duration of muscle activity for serratus anterior was statistically significant among the groups. Post hoc comparison revealed a significantly delayed onset of muscle activation and less duration of muscle activity in the IONP group, and in the WAD group compared to the asymptomatic group. There were no group main effects or interaction effects for upper, middle and lower trapezius.This finding may have implications for scapular stability in these patients because the altered activity in the SA may reflect inconsistent or poorly coordinated muscle activation that may reduce the quality of neuromuscular performance and induce an increased load on the cervical and the thoracic spine.     

Immediate breast reconstruction with tissue expansion

Between October of 1983 and June of 1985, 31 patients underwent primary breast reconstruction with tissue expansion. Tissue expansion was utilized for breast reconstruction when the remaining muscle and skin following modified radical mastectomy was insufficient to accommodate a prosthesis that matched in size and shape the opposite breast. All expanders were placed beneath an investing muscular pocket created by elevating the pectoralis major and serratus anterior. Postoperative expansion began within 1 week, and the breast was expanded to double the volume of the opposite breast. Twenty-two patients have completed their reconstruction with a mean follow-up in 7 months. There were nine complications, including five deflations and four infections. All patients have remained Baker I or Baker II. Creating ptosis to match the breast was accomplished by placing the expander below the rectus fascia and superiorly advancing this expanded tissue at the time of prosthesis placement. Primary breast reconstruction with tissue expansion following modified radical mastectomy is safe, simple, and produces a breast with excellent shape, size, texture, and patient satisfaction.     

Electromyographic amplitude ratio of serratus anterior and upper trapezius muscles during modified push-ups and bench press exercises

Imbalance and weakness of the serratus anterior and upper trapezius force couple have been described in patients with shoulder dysfunction. There is interest in identifying exercises that selectively activate these muscles and including it in rehabilitation protocols. This study aims to verify the UT/SA electromyographic (EMG) amplitude ratio, performed in different upper limb exercises and on two bases of support. Twelve healthy men were tested (average age = 22.8 +/- 3.1 years), and surface EMG was recorded from the upper trapezius and serratus anterior using single differential surface electrodes. Volunteers performed isometric contractions over a stable base of support and on a Swiss ball during the wall push-up (WP), bench press (BP), and push-up (PU) exercises. All SEMG data are reported as a percentage of root mean square or integral of linear envelope from the maximal value obtained in one of three maximal voluntary contractions for each muscle studied. A linear mixed-effect model was performed to compare UT/SA ratio values. The WP, BP, and PU exercises showed UT/SA ratio mean +/- SD values of 0.69 +/- 0.72, 0.14 +/- 0.12, and 0.39 +/- 0.37 for stable surfaces, respectively, whereas for unstable surfaces, the values were 0.73 +/- 0.67, 0.43 +/- 0.39, and 0.32 +/- 0.30. The results demonstrate that UT/SA ratio was influenced by the exercises and by the upper limb base of support. The practical application is to show that BP on a stable surface is the exercise preferred over WP and PU on either surfaces for serratus anterior muscle training in patients with imbalance between the UT/SA force couple or serratus anterior weakness.     

Preservation of projection after reduction mammaplasty: long-term follow-up of the inferior pedicle technique

In reduction mammaplasty by the inferior pedicle technique, the dermal-breast pedicle can be manipulated to form a central breast mound and enhance breast projection. When this technique is applied both to macromastia and breast asymmetry, excellent early results are reported. To study the effects of time on breast reduction, 22 patients were followed for an average of 4.7 years. Contour of the breast mound and projection are well preserved. However, evaluation of long-term results reveals a gradual increase in the inframammary fold to inferior areola distance. Since no increase in the midclavicle to nipple distance is observed, inferior migration of the breast parenchyma and superior displacement of the nipple-areola with respect to the breast mound occur after reduction mammaplasty with the inferior pedicle technique.     

Effect of isometric horizontal abduction on pectoralis major and serratus anterior EMG activity during three exercises in subjects with scapular winging

The aim of this study was to determine the effect of isometric horizontal abduction using Thera-Band during three exercises (forward flexion, scaption, and wall push-up plus) in subjects with scapular winging by investigating the electromyographic (EMG) amplitude of the pectoralis major, serratus anterior and the pectoralis major/serratus anterior activity ratio. Twenty-four males with scapular winging participated in this study. The subjects performed the forward flexion, scaption, and wall push-up plus with and without isometric horizontal abduction using Thera-Band. Surface EMG was used to collect the EMG data of the pectoralis major and serratus anterior during the three exercises. Two-way repeated analyses of variance with two within-subject factors (isometric horizontal abduction condition and exercise type) were used to determine the statistical significance of pectoralis major and serratus anterior EMG activity and the pectoralis major/serratus anterior EMG activity ratio. Pectoralis major EMG activity was significantly lower during forward flexion and wall push-up plus with isometric horizontal abduction, and serratus anterior EMG activity was significantly greater with isometric horizontal abduction. Additionally, the pectoralis major/serratus anterior activity ratio was significantly lower during the forward flexion and wall push-up plus with isometric horizontal abduction. The results of this study suggest that isometric horizontal abduction using Thera-Band can be used as an effective method to facilitate the serratus anterior activity and to reduce excessive pectoralis major activity during exercises for activating serratus anterior.     

Reconstruction of soft-tissue defects using serratus anterior adipofascial free flap

The serratus anterior muscle has been suggested as a versatile and reliable flap for reconstruction of head and neck and extremity injuries. The adipofascial layer overlying the serratus anterior muscle is the anatomic layer, which is supplied by the same branch of thoracodorsal artery. Even though great progress has occurred in the prevention of postoperative adhesion of extremity injuries, the problem has not been completely solved and is still of special importance in complex injuries. Between March of 1995 and February of 1996, seven patients underwent reconstructive operation as a result of soft-tissue defects of the upper or lower extremities or the scalp. We transferred free adipofascial tissue overlying the serratus anterior muscle in three patients and both serratus anterior muscle and adipofascial tissue in four patients. A free adipofascial flap overlying serratus anterior muscle was transferred when a gliding surface was required, owing to the exposure of tendons and neurovascular structures. The average duration from operation to follow-up examination was 8 months (from 4 to 16 months). The results of the operations were satisfactory in functional and cosmetic aspects. This kind of flap was very effective in reconstruction of soft-tissue defects and gliding surfaces for these reasons: easy dissection, the capability of obtaining a long vascular pedicle, large-sized flap, composite flap including muscle or rib, and the fact that there was no serious functional or cosmetic deficit at the donor site.     

The functional anatomy of the shoulder of the savannah monitor lizard (Varanus exanthematicus)

The excursions of the scapulocoracoid and forelimb and the activity of 18 shoulder muscles were studied by simultaneous cineradiography and electromyography in Savannah Monitor lizards (Varanus exanthematicus) walking on a treadmill at speeds of 0.7–1.1 km/hour. During the propulsive phase, the humerus moves anteroposteriorly 40–55° and rotates a total of 30–40°. Simultaneously, the coracoid translates posteriorly along the tongue-and-groove coracosternal joint by a distance equivalent to about 40% the length of the coracoid. Biceps brachii, coraco-brachialis brevis and longus, the middle and posterior parts of the latissimus dorsi and pectoralis, serratus anterior, serratus anterior superficialis, subscapularis, supracoracoideus, and triceps usually become active during the late swing phase and continue activity throughout most or all of propulsion. The anterior part of the latissimus dorsi is active during the transition from propulsive to swing phases. Brachialis, deltoideus scapularis, levator scapulae, the anterior part of pectoralis, scapulo-humeralis posterior, and subcoracoideus are active primarily during the swing phase; they are occasionally active during propulsion. Deltoideus clavicularis, scapulo-humeralis posterior, sternocoracoideus, and the posterior part of the trapezius are biphasic, with activity in both the propulsive and swing phases. A number of shoulder muscles in Varanus exanthematicus and Didelphis virginiana (the Virginia opossum) are similar in attachments, in activity patterns with respect to phases of the step cycle, and in apparent actions. These similarities are interpreted as a pattern inherited from the ancestors of higher tetrapods. The sliding coracosternal joint permits an increase in step length without demanding greater excursion at the shoulder and elbow joints.